1. Field of the Invention
The present invention pertains to method(s) and apparatus for providing a source of radiation and, in particular, for providing a source of radiation for use in digital printing applications.
2. Description of the Prior Art
Semiconductor lasers and laser diodes, in particular, have been popular for nearly a decade now in applications such as optical communications and compact audio disks. Laser diodes are advantageous for use in such applications because of their small size, high efficiency, and remarkable durability. Even so, the more widespread use of laser diodes has been limited by their low power output (typically a few milliwatts), poor spatial quality, and relatively small number of output wavelengths. As a result, their use has largely been limited to applications which do not require a high intensity, uniform source at one of the available output wavelengths.
In printing and copier applications where the output wavelength matches the spectral sensitivity of the medium and available power is sufficient for exposure requirements, laser diodes can be advantageously used because they can be directly modulated at high rates. However, their use has been successful primarily with highly sensitive media.
Consequently, a laser printer or copier, which is to print on a radiation--sensitive or thermally--sensitive recording medium in accordance with a threshold physical phenomenon typically requires a high power source of printing radiation. For example, for a typical recording medium, there is a 1:3 intensity ratio from threshold excitation of the recording medium to its destruction. As a result, the printing radiation is required to be focused onto the recording medium as a spot which has a substantially smooth, uniform intensity thereacross--where a substantially smooth, uniform intensity means that the peak to valley intensity ratio in the spot is smaller, within engineering tolerance, than the threshold to burn-off ratio of the recording medium.
The use of such a thermally sensitive recording medium in a printer prohibits the use of a single laser diode to provide the printing radiation because a single laser diode typically outputs an amount of power in the range between 20 mW to 50 mW, and this amount of power is insufficient to provide proper activation of a typical recording medium. As a consequence, a typical laser printer needs to utilize an array comprised of a multiplicity of laser diodes to provide an amount of radiative power which is suitable for printing. For example, a typical laser printer application might utilize an array comprised of as many as ten laser diodes which are fabricated on the same semiconductor chip. In fact, some reports in the prior art disclose the achievement of 500 mW of power for a laser diode array containing 10 laser diodes which were configured in a linear fashion such as in the configuration shown in FIG. 1. In such a configuration, the near field distribution, i.e., the output spatial intensity distribution within a wavelength or so from the laser diode array, looks like the comb pattern shown in FIG. 2, FIG. 2 being FIG. 4a from U.S. Pat. No. 4,744,616. In addition, the far field distribution, i.e., the output spatial intensity distribution which begins to take shape a distance of several wavelengths from the laser diode array, but defined by convention as the output spatial intensity distribution observed at infinity, looks like the dual lobe pattern shown in FIG. 3, FIG. 3 being FIG. 7 from U.S. Pat. No. 4,791,651. Although the far field distribution contains sufficient power to fulfill the exposure requirements in many printing applications, it is virtually useless for providing a printing beam for a recording medium which requires a substantially uniform radiation exposure for tone control because the energy is substantially concentrated in the dual lobes shown in FIG. 3.
The far field distribution shown in FIG. 3 is produced by an interference phenomenon wherein the output from each of the individual laser diode junctions of the laser diode array adds slightly off-axis and reduces on-axis. This occurs because of cross coupling between the laser diode waveguides, which cross coupling, if it is allowed to dominate the operation of the array, causes adjacent laser diode junctions to emit out of phase by 180 degrees. Because of this interference phenomenon, such a laser diode array is often referred to as a phased laser diode array.
Workers in the prior art have proposed various methods to eliminate the dual lobes in the far field distribution shown in FIG. 3 and to make the spatial intensity pattern more uniform. Specifically, these methods have been addressed to making the laser diode arrays incoherent or randomly phased, i.e., having no definite, regular phase relationship between adjacent laser diode junctions. When a laser diode array is made incoherent, the far field distribution becomes Gaussian, or nearly so, instead of the dual lobed distribution shown in FIG. 3. This is advantageous because a Gaussian distribution can be optically transformed into a "top hat" or "square" distribution which is substantially uniform in intensity, or uniform enough, so that both the uniformity and intensity requirements of a recording medium are satisfied for many area modulation printing applications.
Despite the above, the extra electronic and optical apparatus which are required to provide a "top hat" far field distribution add complexity and expense to a radiation source. Thus, there is a need for a radiation source which utilizes a laser diode array without having to provide a "top hat" far field distribution.